1. Field of the Invention
The present invention relates to a piezoelectric-acoustic transducer, and particularly to a piezoelectric-acoustic transducer used with the object of reducing both costs and power consumption, such as in piezoelectric receivers, piezoelectric speakers, and piezoelectric sounders.
2. Description of the Related Art
Piezoelectric-acoustic transducers are widely used as devices for converting electrical signals to acoustic signals because they enable low power consumption and compact size.
FIG. 1(a) is a sectional view of a piezoelectric-acoustic transducer of the prior art, and FIG. 1(b) is a plan view of the interior of this device. In FIGS. 1(a) and 1(b), diaphragm 102 is supported and secured around the entire circumference of its outer edge inside the unit case 101. Piezoelectric ceramic 107 is attached to the surface of diaphragm 102. By applying voltage to piezoelectric ceramic 107, piezoelectric ceramic 107 bends according to the direction of the applied voltage, bending in an upward direction as shown in FIG. 2(a) or bending in a downward direction as shown in FIG. 2(b), and sound pressure is in turn generated by the bending of diaphragm 102 that occurs with this bending. Materials that may be employed for this diaphragm 102 are limited by the relation between piezoelectric ceramic 107 and thermal expansion coefficients.
With this type of piezoelectric-acoustic transducer, a high level of distortion of the diaphragm is necessary to generate great sound pressure. To this end, Japanese Patent Laid-open No. 227199/88 and Japanese Patent Laid-open No. 227200/88 disclose piezoelectric-acoustic transducers in which distortion of the diaphragm is facilitated by making the periphery of the diaphragm thinner. Such a diaphragm has a piezoelectric bimorph construction in which the diaphragm is composed of two layers of piezoelectric ceramics and is secured and supported around its circumference. Japanese Patent Laid-open No. 227200/88 in particular discloses a construction in which the boundary area between the secured/supported portion and the free-state portion, or the free-state portion adjacent to this boundary, is thinner than the portion in which the bimorph is formed. In Japanese Patent Laid-open No. 227199/88, at least one recessed portion is formed at the boundary between the secured/supported area and the free-state portion, or in the free-state portion adjacent to this area, and by this construction, the circumference of the diaphragm is made thinner.
Japanese Patent Laid-open No. 166717/82 discloses a bimorph piezoelectric oscillator that is constructed by bonding a metal plate to a piezoelectric plate and that is secured at two opposing locations on the periphery of the vibrator with the remaining portion of the periphery left free, thereby increasing the area that functions as a electroacoustic transducer.
However, all of the above-described prior-art piezoelectric-acoustic transducers generate sound pressure through distortion of a diaphragm formed in a single unit with a monolithic piezoelectric unit, and as a result, all suffer from the following drawbacks: First, the conversion efficiency to sound waves is poor and increasing the generated sound pressure is difficult. Not only is there is a limit to the extent to which the periphery of the diaphragm can be thinned in order to facilitate distortion of the diaphragm, but this modification cannot be expected to greatly increase sound pressure. As a second drawback, generated sound waves exhibit a high level of harmonic distortion, and this distortion becomes dramatically worse at sound pressures above a particular level. This problem occurs first because the operation of the diaphragm exhibits a hysteresis characteristic rather than a linear characteristic with respect to the applied voltage, and this characteristic consequently causes irregularity in the phase of generated sound waves. In addition, distortion in sound waves is also caused because the displacement of the diaphragm is limited, and at distortions of the diaphragm greater than a particular level, the displacement becomes nonlinear with respect to input signals.
Finally, a piezoelectric-acoustic transducer of the prior art also has the drawback that the selection of material, weight, and rigidity of the diaphragm is constrained because the diaphragm is formed as a single unit with the piezoelectric that drives the diaphragm, and these constraints limit the degree of design freedom for obtaining ideal acoustic characteristics.